2 research outputs found
Neoclassical tearing modes
Neoclassical tearing modes are one of the most serious concerns for operation on a next-step tokamak device. The modes occur on present tokamaks at normalized pressure (βN) values comparable to those envisaged for baseline scenarios in future devices, such as ITER-FEAT. Further, empirical scalings based on data from many of the present machines point to much lower thresholds on a larger device. However, physics-based models indicate an important role for the seed island mechanisms, which may in fact give rise to increased stability on larger devices - i.e. if the seed island width (required to trigger the NTM) falls below the critical levels required. Fits based on these models suggest this is the case, but are too badly constrained at present to make reliable predictions, and the physics is complex, making quantitative theoretical calculation difficult. Further experiments are required to examine the scaling of the seed, as well as to identify the role and relative sizes of the stabilizing terms that set the critical size for mode growth. In the event that the modes are unavoidable, promising feedback stabilization techniques are being developed with the use of localized RF current drive to change the stability properties of the plasma. Further work is needed to demonstrate sustained access to higher βN and provide data to refine models. This paper reviews the underlying physics and key issues, commenting on the present status of understanding and further work required
Chapter 3: MHD stability, operational limits and disruptions
Progress in the area of MHD stability and disruptions, since the
publication of the 1999 ITER Physics Basis document (1999 Nucl.
Fusion 39 2137-2664), is reviewed. Recent theoretical and
experimental research has made important advances in both understanding
and control of MHD stability in tokamak plasmas. Sawteeth are
anticipated in the ITER baseline ELMy H-mode scenario, but the tools
exist to avoid or control them through localized current drive or fast
ion generation. Active control of other MHD instabilities will most
likely be also required in ITER. Extrapolation from existing experiments
indicates that stabilization of neoclassical tearing modes by highly
localized feedback-controlled current drive should be possible in ITER.
Resistive wall modes are a key issue for advanced scenarios, but again,
existing experiments indicate that these modes can be stabilized by a
combination of plasma rotation and direct feedback control with
non-axisymmetric coils. Reduction of error fields is a requirement for
avoiding non-rotating magnetic island formation and for maintaining
plasma rotation to help stabilize resistive wall modes. Recent
experiments have shown the feasibility of reducing error fields to an
acceptable level by means of non-axisymmetric coils, possibly controlled
by feedback. The MHD stability limits associated with advanced scenarios
are becoming well understood theoretically, and can be extended by
tailoring of the pressure and current density profiles as well as by
other techniques mentioned here. There have been significant advances
also in the control of disruptions, most notably by injection of massive
quantities of gas, leading to reduced halo current fractions and a
larger fraction of the total thermal and magnetic energy dissipated by
radiation. These advances in disruption control are supported by the
development of means to predict impending disruption, most notably using
neural networks. In addition to these advances in means to control or
ameliorate the consequences of MHD instabilities, there has been
significant progress in improving physics understanding and modelling.
This progress has been in areas including the mechanisms governing NTM
growth and seeding, in understanding the damping controlling RWM
stability and in modelling RWM feedback schemes. For disruptions there
has been continued progress on the instability mechanisms that underlie
various classes of disruption, on the detailed modelling of halo
currents and forces and in refining predictions of quench rates and
disruption power loads. Overall the studies reviewed in this chapter
demonstrate that MHD instabilities can be controlled, avoided or
ameliorated to the extent that they should not compromise ITER
operation, though they will necessarily impose a range of constraints